Electron energy barriers between the valence band of (100)Si and the conduction bands of ultrathin SiO2, Al2O3, ZrO2 insulators and their stacks were determined using internal photoemission of electrons. For SiO2, the barrier of 4.25 +/-0.05 eV was found unchanged down to the oxide thickness of approximate to1 nm. The barriers for Al2O3 and ZrO2 are substantially lower: 3.25 +/-0.08 and 3.1 +/-0.1 eV, respectively. Thermal oxidation at 650-800 degreesC enhances the barriers at the Si/Al2O3 and Si/ZrO2 interfaces but does not reduce the high density of band tail states in the insulators, suggesting the formation of silicates. (C) 2001 American Institute of Physics.